A conventional induction heating cooking device that induction-heats a load made of nonmagnetic metal with low resistivity, is disclosed in Japanese Patent Unexamined Publication No. 2002-75620, for example.
FIG. 7 is a circuit diagram of the conventional induction heating cooking device. In FIG. 7, power supply 21 is a 200 V commercial power supply, namely a low frequency alternating-current power supply, and is connected to an input terminal of rectifying circuit 22 with a bridge diode. First smoothing capacitor (hereinafter referred to as “capacitor”) 23 is connected between the output terminals of rectifying circuit 22. A series connection body of chock coil 24 and second switching element (insulated gate bipolar transistor (IGBT)) (hereinafter referred to as “element”) 27 is also connected between the output terminals of rectifying circuit 22. Heating coil 29 faces load 31 such as an aluminum-made pan.
The low-potential-side terminal (emitter) of second smoothing capacitor (hereinafter referred to as “capacitor”) 32 is connected to a negative electrode terminal of rectifying circuit 22. The high-potential-side terminal of capacitor 32 is connected to the high-potential-side terminal (collector) of first switching element (IGBT) (hereinafter referred to as “element”) 25. The low-potential-side terminal of element 25 is connected to a connection point between the high-potential-side terminal (collector) of element 27 and chock coil 24. The series resonant circuit of heating coil 29 and resonant capacitor 30 is connected to element 27 in parallel.
First diode (hereinafter referred to as “diode”) 26 (first inverse conducting element) is connected to element 25 in anti-parallel. The cathode of diode 26 is connected to the collector of element 25. Second diode (hereinafter referred to as “diode”) 28 (second inverse conducting element) is connected to element 27 in anti-parallel. Namely, the cathode of diode 28 is connected to the collector of element 27. Controlling means 33 outputs signals to gates of elements 25 and 27 so as to produce a predetermined output.
In the induction heating cooking device having this constitution, the frequency of resonance current is set twice or more as high as the driving frequency of elements 25 and 27. Chock coil 24 increases the voltage of smoothing capacitor 32, so that a nonmagnetic load with low resistivity such as aluminum is induction-heated with a high output power.
When the resonance frequency is set substantially 2N (where, N is a positive integer) times higher than the driving frequency of switching elements in the conventional configuration, however, switching element driving duty defined by rates of the driving periods of element 25 and element 27 for maximizing the heating output is not 0.5. The on-state loss of each of switching elements 25 and 27 depends on each on-state period, so that imbalance between the losses occurs. Thus, especially when the heating output is large, it is difficult to cool the switching elements.